Warren defeated Scott Brown in 2012

Sen. Elizabeth Warren is throwing her fundraising muscle behind fellow Democratic Sen. Jeanne Shaheen, who's facing a challenge from former Massachusetts Republican Sen. Scott Brown as she seeks a second term in New Hampshire.

Warren defeated Brown in a 2012 race that was the costliest in Massachusetts history. In a recent email, Warren thanked supporters for helping her raise $40,000 for Shaheen over the weekend.

In the email, Warren also faults Brown for failing to agree to Shaheen's challenge to sign a so-called "people's pledge" on outside money in the New Hampshire race, saying he's "signed on to get the dirtiest, most secretive help he can find to attack Jeanne Shaheen."

"He spends a lot of time talking about how he's an outsider, but Scott Brown welcoming Karl Rove and the Koch brothers with open arms is the ultimate insider move," Warren said in the email.

Brown and Warren signed a similar pledge in 2012, dramatically tamping down the amount of outside money spent on television, radio and internet ads.

Brown, who's formed an exploratory committee, hasn't said whether he would agree to the pledge but on Saturday noted Shaheen was on a West Coast fundraising swing he said would provide money to third-party groups for more outside negative ads against him.

"It's hard to view Jeanne Shaheen's actions as anything other than hypocritical and self-serving," Brown said. "The people of New Hampshire can see through the Washington-style game she is playing."

American Crossroads, a group with ties to veteran GOP strategist Karl Rove, is spending $650,000 on a new television advertising campaign against Shaheen. Before Brown entered the race, outside groups on both sides spent more than $1 million on television ads.

Shaheen said the pledge would require both candidates to donate half the cost of any outside group advertising campaign to a charity of the opposing candidate's choosing.

That mirrors the original pledged hammered out by Brown and Warren. In that deal, each campaign agreed to donate half the cost of any third-party ad to charity if that ad either supported their candidacy or attacked their opponent by name.

Brown was the first to lay out the basic terms of the 2012 deal and hailed what he dubbed the "people's pledge" as a historic breakthrough.

"This is a great victory for the people of Massachusetts, and a bold statement that puts Super PACs and other third parties on notice that their interference in this race will not be tolerated," Brown said at the time.

Before the deal was signed, outside groups - including the League of Conservation Voters, the League of Women Voters and Crossroads GPS, an affiliate of American Crossroads - had spent millions on attack ads. Even with the deal, outside groups spent millions on non-prohibited activities, including mass mailings, door-to-door canvassing and automated phone calls.

The race ended up being Massachusetts' most expensive, with both candidates spending tens of millions.

Warren, in her second year in the Senate, is quickly becoming a Democratic fundraising powerhouse.

Warren's political action committee reported raising more than half a million dollars last year, money she says will go to trying to block a Republican takeover of the Senate.

According to documents filed with the Federal Elections Commission, Warren's PAC for a Level Playing Field took in more than $582,839 in contributions in 2013.

The PAC doled out $257,000 to other congressional candidates and party committees, including virtually every member of Massachusetts' all-Democratic congressional delegation and other Democratic senators like Al Franken of Minnesota, Mark Pryor of Arkansas and Shaheen.

Warren's PAC also made donations to other Democratic PACs, including $15,000 to the Democratic Senatorial Campaign Committee, $15,000 to a related DSCC fund, and $5,000 to the Massachusetts Democratic State Committee's federal fund.

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